Field Study Assignment for Geology 103 at Lake Tahoe Community College.

1. South Lake Tahoe
ADVENTURES IN TIME
Field Study
Geology 103
Prof. Mark Lawler
8/3/2014
Lake Tahoe Community College

2. Adventures in Time
The Lake Tahoe Basin is a treasure chest of evidence of all manner of geologic history. Here we can find traces
of:
 Accretion
 Magmatic Intrusions
 Uplift and Extension tectonics
 Volcanism
 Glacial scouring
 Fauna
Unless otherwise noted, all material comes from lecture notes from Scott Valentine, LTCC 2013, 2014, or, my
own observations.
Photo credits are mine, unless otherwise noted.

3. Accretion
 Accretion is a process by which material is added to a tectonic plate or a
landmass. This material may be sediment, volcanic arcs, seamounts or other
igneous features (Valentine, 2014).
 Much of the western half of North America is made up of accretionary material
that was welded or sutured onto the craton and platform.
 We find evidence of this in some of the rocks in the basin, most of which can be
found in the broken debris left on the glacial moraines.
 The following pictures will show some of the rock types discovered by this ardent
explorer. 

4. Accretionary Rocks: Chert
 Chert is a metamorphic rock formed in association
with an accretionary event.
 Hard and brittle like flint, chert fractures
conchoidally and was used by early humans for
cutting tools.
 Chert is a microcrystalline or cryptocrystalline
sedimentary rock material composed of silicon
dioxide.
 Some of the SiO2 in chert has a biological origin.
Shallow seas were home to diatoms and
radiolarians, which had a glassy silica skeleton.
When these organisms died their silica skeletons
fell to the bottom, dissolved, recrystallized and
become part of a chert nodule. Chert formed in
this way could be considered a biological
sedimentary rock.

5. Accretionary Rocks: Schist (Blue)
 High-pressure, low-temperature conditions are
most typical of subduction, where marine crust
and sediments are carried beneath a
continental plate and kneaded by changing
tectonic motions while sodium-rich fluids
marinate the rocks.
 This example seems to be made from sodium-
rich mafic rocks like basalt and gabbro.
 Schist is somewhat rare in the Sierra Nevadas,
and this sample seems to affirm the notion
that this area was once covered by a shallow
sea.

6. Magmatic Intrusions
 Due to the subduction of the ancient Farallon Plate by the North American Plate, an up
welling of magma occurred in the form of Plutons.
 As the magma came into contact with “country rock”, contact metamorphism occurred
and roof pendants were created.
 The many plutons of magma formed the Sierra Batholith (Collection of plutons).
 The magma plutons cooled slowly underground forming granodiorite “balloons”.
 The uplift of the Sierras increased the rate of erosion and eventually the granitic
“balloons” were exposed.
 We see these plutons exposed today throughout the Lake Tahoe Basin.
(Valentine, 2014).

7. Magmatic Intrusions: Tallac Medisedimentary Rock
 The “roof pendant” from magma interactions
with country rock (sedimentary) yield a
metamorphic rock called Tallac
metasedimentary.
 Metasedimentary denotes that parent
material was sedimentary.
 Note lines of dark and white material
represent layers, most likely mud and ash, or
possibly sandstone.
 The rust on the rock is oxidized ferrous
material. The mud layers are iron rich, just as
ocean mud is expected to be.

8. Magmatic Intrusions: Granodiorite
 Grandodiorite is an intrusive rock of the
granitic family, and is made up of quartz,
feldspars, micas and biotites.
 Granitic rocks are formed from slowly
cooling magma intrusions deep
underground.
 Inclusions are bits of country rock that have
fallen into the intruding magma chamber
from the top of the chamber, usually.
 Magenta arrows point to inclusions.

9. Magmatic Intrusions: Inclusions
 Andesite with inclusions was a new one for me.
 Inclusions are bits of country rock that have
fallen into the intruding magma chamber from
the top of the chamber, usually.
 This specimen comes from a bed of rock that
suffered through periods of glaciation, erosion
and thermal cracking (frost),located on a trail
between Fallen Leaf and Lily lakes.

10. Uplift & Extensional Tectonics
 The Sierra Nevada Mountains were uplifted due to compressional tectonics
approx. 100 ma, as the leading edge of the North American Plate “lurched” over
the subducting ancient Farallon Plate.
 The crust behind the leading edge was stretched, extended and caused normal
faulting to occur, forming the Basin & Range geologic province.
 The Lake Tahoe Basin was initially formed during an extensional episode as the
“block” dropped forming a graben.
 The Tahoe Basin was not formed in one singular event, thus we find evidence in
scarps and escarpments.

11. Uplift & Extensional Tectonics: Grabens
 Horsts and Grabens are formed
from normal faulting, and
associated with extensional
tectonics.
 The crust extends (stretches),
allowing some blocks to drop,
while others may uplift.
 The Tahoe basin is a graben.
 Locally, the Sierras and the
Carson Range would be
considered horsts.
Tahoe
Basin
Sierra
Nevadas
Carson
Range
Photo Credit: MarsSed, 2010

12. Volcanism
 The Lake Tahoe Basin is centrally located in the Walker Lane Deformation Zone,
which is active today.
 Walker Lane Deformation Zone was volcanically active throughout the Cenozoic,
producing several volcanoes in the area surrounding Tahoe.
 Mount Pluto, and extinct volcano today, erupted approx. 35 ma and it’s lava flows
blocked off the north end of the Tahoe Basin.
 The main rock evidence of the volcanic activity is Andesite and Basalt, and,
occasionally a piece of obsidian.

13. Volcanism: Andesite
 Soils of the Tahoe basin come primarily from
andesitic volcanic rocks and granodiorite, with
minor areas of metamorphic rock.
 Andesite is an extrusive igneous, volcanic rock, of
intermediate composition, with aphanitic to
porphyritic texture.
 This andesite probably originated from volcanoes
of the Walker Lane Deformation Belt ~30 Ma.
 Note that Lake Tahoe is within the Walker Lane
Deformation Belt.

14. Glacial Scouring
 The Tahoe Basin has been sculpted by several periods of glacier activity, mainly the Tioga and
Tahoe glaciations.
 The Tioga Glaciation occurred approx. 25 ma.
 The Tahoe Glaciation occurred approx. 10 ma.
 Glaciers formed after the Sierran uplift occurred as the higher mountains attracted more moisture
and cooler temperatures.
 Cirques are evidence of places glaciers first formed.
 Moraines are the debris piles left behind marking a glacier’s passage and end.
 Erratics are boulders that were “plucked” by glaciers and left behind when the glaciers
melted/retreated.
 Striations and polishing occur when a glacier moves across harder rock surfaces, leaving shallow
grooves and in some places polished or smooth surfaces.

15. Glacial Scouring: Angora Cirque
 The “bowl” carved out of the
side of Angora Mountain &
Echo Peak is a Cirque, the
original home of the young
glacier.
 The glacier would then move
down, carving and bulldozing
as it grew.
 Within the cirque are small
lakes, called tarns.
 The Angora cirque was formed
during the Tahoe Glaciation.
 The Tahoe reached its
maximum extent perhaps
about 70,000 years ago.

16. Glacial Scouring: Striations & Polish
 Glaciers leave their mark on the
local landscape in the form of
striations (grooves cut into the
rock) and polished surfaces.
 Purple Arrow indicates striations
left from glacial movement.
 Magenta arrow indicates area of
glacial polish.
 These photos are from the Glen
Alpine glacial area on the south
end of Fallen Leaf Lake, nearer to
Lily Lake.

17. Fauna of the South Lake Tahoe Basin
 The South Lake Tahoe Basin is home to several interesting species which might
include:
 Beavers
 Bald Eagles
 Chickarees
 Coyotes
 Bears

18. Fauna: Beavers
 Beavers were native in the area until
trapping and hunting virtually
eliminated them.
 Beavers were reintroduced to the area
in 1923.
 Beavers thrive along the Upper
Truckee River and several other larger
creeks and streams.
 Beaver dams are vital in helping to
return the clarity of Lake Tahoe to its
former grandeur.
 Beavers have long fossil history in the
Northern Hemisphere beginning in
the Eocene, and many species of giant
beaver existed until quite recently,
such as Castoroides in North America.

19. Fauna: Bald Eagles
 Bald Eagles can be viewed at Baldwin Beach,
Taylor Creek and several other places along
the lake.
 All birds - the class Aves - arose from reptiles.
Whether they came from crocodile-like
thecodonts, which are ancestors of dinosaurs,
or from small dinosaurs themselves is still
debated in the scientific community
(Weidensaul, 1996).
 The first modern raptors, whose diurnal
members make up the order Falconiformes,
appear in the fossil records from the Eocene
epoch about 50 million years ago (Stalmaster,
1987).
 The sea eagles of this genus probably came
from the scavenging Asian and Australian
kites of genus Haliastur, and the first sea
eagle appears 25 million years ago
(Stalmaster, 1987).

20. Fauna: Chickaree
 Four types of squirrels are abundant in the South Lake
area.
 Ground Squirrel
 Golden Mantled Squirrel
 Grey Squirrel
 Chickaree (Douglas Squirrel)
 Chickarees are descended from the oldest squirrel
fossil, Hesperopetes, dating back to the late Eocene,
~40–35 million years ago (Emry, 2007).
 The common ancestor of all squirrels, living and fossil,
lived in North America, as these three most ancient
lineages seem to have radiated from there; if squirrels
had originated in Eurasia, for example, one would
expect quite ancient lineages in Africa, but African
squirrels seem to be of more recent origin(Steppan,
2004).
 Interestingly, squirrels and beavers are related.

21. Fauna: Tamarack Pine (Lodgepole)
 Tamarack, or Lodge Pole Pines (Pinus contorta
murrayana) are descendants of ancient
gymnosperms.
 It is widely accepted that the gymnosperms
originated in the late Carboniferous period
(Campbell & Reece, n.d.).
 This appears to have been the result of a whole
genome duplication event ~300 million years ago
(jiao, 2011).
 Early characteristics of seed plants were evident in
fossil progymnosperms of the late Devonian
period around 380 million years ago (Jiao, 2011).
 This tree has many uses, such as shelter (tipis),
medicinal (from the bark), both internally and
externally.

22. Evolution of the Tahoe Basin
 There is scanty fossil record within the southern Tahoe Basin, as most rock is
metamorphic or intrusive. Fossils are usually found in sedimentary rock
formations, and this sort of formation just doesn’t exist in the Tahoe Basin.
 The sediments of the Tahoe Basin are thought to contain a continuous 1-million-
year record of climate, one of the longest on the continent. Fossil remains of
invertebrates and fish scales portray the postglacial history of Lake Tahoe,
beginning about 11,000 years ago. More recent sediment layers preserve sawdust
from the Glenbrook sawmills of the Gold Rush, chronicle the introduction of tetra
ethyl lead in gasoline in 1948, and record the appearance of mercury from
industrial atmospheric pollution (Goldman, 2006).

23. References
 Campbell and Reece; Biology, Eighth edition
 Cenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new model for lithosphere degradation. (n.d.). Retrieved
from http://geosphere.gsapubs.org/content/8/2/265.abstract
 Emry, R. J.; Korth, W. W. (2007). "A new genus of squirrel (Rodentia, Sciuridae) from the mid-Cenozoic of North America". Journal of
Vertebrate Paleontology 27 (3): 693. doi:10.1671/0272-4634(2007)27[693:ANGOSR]2.0.CO;2.
 Goldman, C. R. (2006, April). California Agriculture Online. Retrieved August 5, 2014, from
http://californiaagriculture.ucanr.org/landingpage.cfm?articleid=ca.v060n02p45
 iao Y, Wickett NJ, Ayyampalayam S, Chanderbali AS, Landherr L, Ralph PE, Tomsho LP, Hu Y, Liang H, Soltis PS, Soltis DE, Clifton SW,
Schlarbaum SE, Schuster SC, Ma H, Leebens-Mack J, Depamphilis CW (2011) Ancestral polyploidy in seed plants and angiosperms.
Nature
 Lectures by Scott Valentine, LTCC, 2013, 2014
 MarsSed 2010 Field trip – Day 1: Guadalupe Mountains and Evaporites | The Martian Chronicles. (n.d.). Retrieved from
http://martianchronicles.wordpress.com/2010/04/26/marssed2010-field-tri-day1/ (Photo Credit)
 Stalmaster, Mark V. 1987. THE BALD EAGLE. University Books, New York.
 Steppan, S. J.; Storz, B. L.; Hoffmann, R. S. (2004). "Nuclear DNA phylogeny of the squirrels (Mammalia: Rodentia) and the evolution of
arboreality from c-myc and RAG1". Molecular Phylogenetics and Evolution 30 (3): 703–719. doi:10.1016/S1055-7903(03)00204-5. PMID
15012949.
 Weidensaul, Scott. 1996. RAPTORS: THE BIRDS OF PREY.Lyons and Burford Publishers.